Sphincter muscles, when contracted, close natural body openings. When they do not contract normally, unintended leakage of body fluids and solids can cause severe discomfort, pain and/or embarrassment.
Two examples are the lower esophageal sphincter muscles at the distal portion of the esophagus and the anal canal sphincter system. The esophageal sphincter muscle system acts as a barrier preventing reflux of acidic gastric contents. When it fails, there is an uncomfortable or painful acid condition in the esophagus and throat. The anal sphincter muscle system also acts as a barrier preventing uncontrolled passage of fecal contents from the rectum. The resulting fecal incontinence causes embarrassment and can severely limit a person's activities.
Fortunately, some sphincter muscle problems can be improved by surgery or by medical treatment. The anal sphincter muscle system includes both internal and external sphincter muscles. The internal muscles are involuntary and the external muscles include two major muscle groups with both voluntary and reflex responsiveness.
One of the conventional tools available for the diagnosis and also the treatment of some causes of anal incontinence is a multilumen manometric catheter for measuring the contracting pressure of the sphincter muscles around the anal canal. The conventional catheter comprises a plurality of pressure-sensing tubes (or a unitary plastic extrusion with individual lumina instead of separate tubes). These tubes or lumina extend in a circular array about a central inflation tube through which a rectal balloon at the distal end is inflated. Side openings in the water-perfusion tubes are provided at different axial levels or positions to transmit canal pressures from those different levels. Pressure readouts are obtained from coupling at the proximal ends of the tubes using a conventional hydraulic capillary infusion device, pressure transducers, and a graphic recorder.
With the catheter device and readout equipment in place, canal pressures are determined by having the patient relax, with and without the rectal balloon inflated, and also exert voluntary contraction efforts. These voluntary contraction efforts can be sustained for only short times during which multiple pressure readings at the various levels along the catheter can be read to determine which muscles are effective and to what extent.
Unfortunately, there are serious drawbacks using this procedure with conventional catheters. The pressure readings are imprecise because it is impossible to obtain instant comparative pressure readings in the same quadrant at different axial levels along the canal, and it is impossible to obtain a reliable three dimensional pressure profile at any one instant in time. The most serious drawback is that, as shown in FIG. 12A, in the pressure-sensing section of the conventional prior art catheter, the water-perfusion tubes 20 are straight and parallel to the longitudinal axis X--X. Inasmuch as there cannot be more than one side opening 22 in each straight tube, and the side openings 22 are circumferentially displaced from one another at different levels along the longitudinal axis, the only practical way they can be positioned at different levels is to arrange them in a helical line about the catheter as is shown in FIG. 12A. It is impossible to obtain a series of simultaneous pressure readings in one quadrant along any straight line parallel to the catheter axis because each pressure reading location (side-hole) 22 is offset circumferentially from adjacent pressure reading side-holes. Thus, the eight side-openings 22 shown in FIG. 12A comprise a series of holes 1, 2, 3, 4, 5, 6, 7, and 8 disposed in a helical pattern about the catheter. No two holes are at the same axial level, or in a same line parallel to the catheter axis.
Another problem in obtaining meaningful pressure readings with the conventional prior art catheter is that the sphincter muscle voluntary contraction force can be maximally generated momentarily before some weakening occurs. The catheter therefore cannot be rotated during a contraction for the purpose of taking meaningful pressure readings along any straight line parallel to the axis. Additionally, any motion of the catheter during a squeeze can set off involuntary reflex contractions which would interfere with the normal voluntary contraction pattern.
Another consideration making it impossible to obtain meaningful comparative pressure readings at different levels with the conventional straight tubes 20 shown in FIG. 12A is that the external anal sphincter muscles are asymmetric as will be described in connection with FIGS. 8, 9 and 10. They do not exert uniform contractile forces. They are loops, exerting oppositely directed lateral forces against the canal. The top loop sometimes referred to as the puborectalis and deep portion of the external sphincter, is connected anteriorly to the pubic bone and pulls the upper portion of the canal forwardly. The lower muscle group is a loop connected posteriorily to the coccyx (tail bone) and pulls the lower half of the canal rearwardly. When both muscle groups are contracted normally, pressures should maximize at an upper posterior location and at a lower anterior location and should be generally the same in anterior, posterior, right and left lateral locations at the intermediate level where the two loops overlap.
Considering the asymmetric configurations of the muscles, and the inability of a patient to voluntarily contract these muscles for any long time period, it would be advantageous if the catheter used for pressure measurements could measure pressures simultaneously, at different levels, in the same vertical plane.
Only one serious attempt to develop such an instrument is known. That was a multiple strain gauge instrument described in Gut, Volume 10, 1969, pages 160-163. This was a multiple electromechanical strain gauge which was unreliable and broke down after minimal use.